The present invention relates to an image forming apparatus, such as a copying machine, a printer or a facsimile machine, which performs electrophotographic image forming operation.
Many of currently used image forming apparatuses including copying machines, printers and facsimile machines employ an electrophotographic image forming process for reproducing image information on such recording media as sheets of paper. Generally, the electrophotographic image forming process includes a charging stage in which a surface of an image carrying member, or a photosensitive drum, is charged to a specific high surface potential, an exposure stage in which an electrostatic latent image is. formed on the surface of the image carrying member by exposing the surface to light controllably projected thereto based on image information to produce varying surface potentials, a development stage in which the latent image is converted into a visible toner image by supplying toner particles onto the surface of the image carrying member, an image transfer stage in which the toner image on the surface of the image carrying member is transferred onto a surface of a recording medium, and a fixing stage in which the transferred toner image is fused onto the surface of the recording medium.
Traditionally employed in the aforementioned charging stage of the electrophotographic image forming process has been a conventional charging method in which a high voltage is applied to a main charger disposed face to face with the surface of the image carrying member to produce a corona discharge. This conventional charging method poses a problem related to environmental degradation due to the influence of ozone produced as a byproduct of the corona discharge. In addition, there is a growing demand today for a reduction in power consumption. Under these circumstances, a contact charging method which uses a charging roller, a charging brush or the like has been proposed in recent years as disclosed in Japanese Laid-open Patent Publication No. 2001-109235, for instance.
In the exposure stage, a digital exposure method is often used today as a result of development of office automation equipment including computers instead of an analog exposure method in which an image carrying member is exposed to light projected to and reflected from an original placed on a platen glass and guided to the image carrying member through multiple mirrors and a through lens. In a digital exposure process, image information picked up by an image scanning section or transmitted from one of terminal devices through a network, to which the image forming apparatus is connected, is once stored in a control section of the image forming apparatus and subjected to image processing. The image carrying member is then exposed to light modulated by the processed image information in an exposure unit (e.g., a laser scan unit).
Japanese Laid-open Patent Publication Nos. H05-040381 and H08-248648 disclose another exposure method developed to cope with a demand for a image forming apparatus of reduced size. The exposure method disclosed in the Publications is a so-called backside exposure method in which charging, exposing and developing operations are simultaneously performed by use of a cylinder-shaped transparent photosensitive drum. In an image forming process adopting the backside exposure method, the photosensitive drum is exposed to light modulated by image information from inside its cylindrical structure to form an electrostatic latent image on the photosensitive drum, and the latent image is developed as electrically conductive toner particles are attracted to exposed surface areas of the photosensitive drum from its outside.
More specifically, an outer surface of the image carrying member is locally charged by static charges of electrically conductive toner at a first half portion of a developing nip area where the outer surface of the image carrying member moves along the electrically conductive toner with friction, whereas image writing light is projected onto an inner surface of the image carrying member to form an electrostatic latent image on the outer surface of the image carrying member so that the toner particles are attracted to the exposed surface areas (or the latent image) on the photosensitive drum at a second half portion of the developing nip area to form a visible toner image.
Another conventionally known image forming process is introduced in an article titled “Direct Formation of Electrostatic Latent Image by Means of Photoelectric Emission” published in Journal of Institute of Electrostatics Japan (IEJ) 1999 (Vol. 23 No. 3). This direct imaging process employs a xenon light source which projects light modulated by image information onto a photoelectric surface. When illuminated by the light, the photoelectric surface emits electrons toward a surface of an image carrying member to write the image information thereon.
The aforementioned conventional image forming processes have their inherent drawbacks, however. While the contact charging method serves to reduce the amount of ozone produced in the charging stage, there arises the need to rotate the charging roller or charging brush in a controlled fashion and it is not possible to sufficiently reduce a charging voltage compared to a case where a charger is used to charge the image carrying member. In addition, while the image carrying member continuously turns during the image forming process and the surface of the image carrying member repetitively undergoes the charging, exposure, development and transfer stages, the toner supplied to the surface of the image carrying member is not transferred in its entirety to the surface of the recording medium in the image transfer stage, but part of the toner that is left on the surface of the image carrying member and is attracted to the charging roller or the charging brush. Residual toner particles attracted to the charging roller or the charging brush become loose when a voltage is applied in a succeeding charging step and, as a consequence, the toner particles firmly adhere to the charging roller or the charging brush. This phenomenon could damage the surface of the image carrying member and cause eventual degradation of image quality.
In either of the aforementioned conventional analog and digital exposure methods, it is necessary to configure a light path including the focal length of an optical system for focusing image writing light on the surface of the image carrying member. For this reason, the optical system must have a high accuracy and the need for such a light path makes it difficult to achieve compact design of the image forming apparatus. Particularly in the digital exposure method employing a laser scan unit, it is necessary to rotate a polygon mirror for redirecting a laser beam at a high speed. Thus, the digital exposure method is associated with such technical problems as difficulties in precisely controlling high-speed rotation of the polygon mirror and the need for a dustproof structure for preventing a whirl of dust which might be produced by air currents caused by the rotating polygon mirror. These problems could result in an inability to achieve compact design as well as degradation of image quality.
One problem of the aforementioned backside exposure method is a difficulty in choosing material of a transparent cylinder used as the image carrying member. Another problem of the backside exposure method is that considerably high accuracy is needed in installing a driving mechanism to ensure proper charging of the image carrying member, writing of the image information and development of the visible toner image, because the charging of the image carrying member, the writing of the image information and the development of the visible toner image are performed within a developing gap, which normally measures about 2 mm to 5 mm, where the image carrying member comes into contact with toner particles. Inadequate installation accuracy of the driving mechanism significantly affects the image quality. Since the outer surface of the image carrying member is charged by use of the electrically conductive toner in the backside exposure method, it is necessary to apply a relatively high voltage to the electrically conductive toner by means of a developing sleeve. Considerable variations occur in potential to which the electrically conductive toner is charged as a result of voltages applied thereto. The toner is apt to deteriorate quickly due to such variations in potential.
The aforementioned direct imaging approach introduced in the article in the IEJ Journal also has problems from a practical viewpoint. Specifically, the direct imaging approach is likely to increase the physical size of an image forming apparatus and pose a problem with respect to a method of converging light in a light source section. While the article discloses a flat-type plotter as a practical example of application of the direct imaging approach, the recording medium is limited in size by the size of a dielectric layer on which an electrostatic latent image is formed and, therefore, the approach of the article can not be applied to ordinary image forming apparatuses which can selectively form images on recording media having different sizes. In addition, the dielectric layer must be cleaned upon completing an image forming step for each single image before proceeding to a next step. For this reason, the image forming apparatus employing the direct imaging approach can form a limited number of images per unit time and is not quite suited to image forming operation in which a large number of images need to be processed.